This invention relates generally to an electronic circuit. More particularly, it related to a sense amplifier and related circuitry.
Electronic circuits are used for various applications, including digital logic. Digital logic circuits are used in memory devices, microprocessors, controllers, digital signal processors, application specific integrated circuits, and others. Digital logic circuits can be classified into three popular logic families: (1) transistor-transistor logic (TTL), (2) emitter-coupled logic (ECL), and complementary metal-oxide-semiconductor (CMOS). CMOS logic circuits offer advantages over other logic families because of its low dissipation power, compact design, and noise immunity.
In the area of memory devices, a variety of volatile and non-volatile memory devices have been used or proposed by the electronic industry. Example of such memory devices include random access memory (RAM), read only memory (ROM), programmable read-only memory (PROM), erasable PROM (EPROM), and flash memory devices. These memory devices can be implemented in a dedicated memory integrated circuit or incorporated within a processor, such as an embedded processor.
Memory devices typically include a memory array having M rows by N columns of memory cells, a row decoder, a column decoder, sense amplifiers, and output buffers. The row and column decoders accept input address lines and generate control signals, corresponding to the address lines, that are used to select the desired memory cells from which to read and to which to write. During a read operation, the selected memory cells are coupled to the associated sense amplifiers that detect the state (i.e., logic high or logic low) of the memory cells. The signals from the sense amplifiers are receive by the output buffers, latched, and provided as the output logic signals.
Typically, the state of a memory cell is determine by a "voltage" or a charge on a floating gate of a transistor that makes up the memory cell. The voltage determines the conduction state of the memory cell. For an N-channel enhancement mode transistor memory cell, a high voltage on the floating gate causes the transistor to conduct current when selected. Alternatively, a low voltage at the floating gate causes the transistor not to conduct when selected. Typically, the transistor is sized such that only a small amount of current passes through a conducting transistor.
A sense amplifier is coupled, usually through a multiplexer, to the selected memory cell and determines whether the selected memory cell has been "programmed" to conduct or not to conduct. An example of a sense amplifier design is disclosed in U.S. Pat. No. 5,386,158 entitled "SENSING CIRCUIT FOR A FLOATING GATE MEMORY DEVICE", issued Jan. 31, 1995. The sense amplifier of the '158 patent compares a voltage of the selected memory cell against a voltage on a "dummy" memory cell that is connected in a conducting state. The sense amplifier includes two voltage amplifiers and two current mirrors one voltage amplifier and current mirror for each of the two memory cells. The voltage amplifier for the selected memory cell includes a "control" transistor having its drain connected to its gate. This "control" transistor provides additional gain for the voltage amplifier. The sense amplifier provides an output signal indicative of the logic state of the selected memory cell.
The performance of many sense amplifiers is susceptible to temperature and power supply variations. For example, the sense amplifier of the '158 patent includes a diode 34 in the voltage amplifier 20. At high temperature and low power supply voltage, the diode slows down its charging of a pull up transistor 38, thereby noticeably slowing down the sense amplifier.
The ability to correctly "read" the values stored in the memory cells is dependent, in part, on the performance of the sense amplifiers. Therefore, sense amplifiers having improved performance and less susceptible to component tolerance, temperature, and power supply variations are clearly desired